Shoe press belt

ABSTRACT

In a shoe press belt, in which the nip zone is narrower than the shoe width, the shearing force which normally acts on the belt at the outer boundary of the nip zone is made either weak or non-existent by forming the outer portions of the belt of less hardness than the inner, or central, portion of the belt, so that the less hard portions can be positioned directly opposite the outer boundaries of the nip zone. Alternatively, the outer portions of the belt can be made thinner than the central portion of the belt and the thinner portions positioned directly opposite to the nip zone boundaries. The shearing force applied to the belt at the nip ends of the roll is reduced or eliminated, cracking resulting from the shear force is reduced or avoided, and a belt having improved durability results.

FIELD OF THE INVENTION

[0001] This invention relates to a belt for a shoe press in the presspart of a papermaking machine. In particular, the invention relates toimprovements in the durability of a shoe press belt.

BACKGROUND OF THE INVENTION

[0002] Shoe presses used in recent years in the press parts ofpapermaking machinery are classified roughly into the two types shown inFIGS. 8 and 9. Each of these shoe presses includes a press roll P and ashoe S. The shoe S has an arcuate top recess conforming with the bottomof a roll P. An endless upper felt 92 and an endless lower felt 93 runbetween the roll P and the shoe S, and a wet web W runs between thefelts 92 and 93. An endless press belt B runs between the lower felt 93and the shoe S, and follows the felt 93. The shoe S urges the belt Bupward to press the felts 92 and 93 against the roll P. The effect ofthe shoe is to form a wide nip zone, which improves the water squeezingeffect of the pressure exerted between the roll P and the shoe S.

[0003] The press belt B of the shoe press 91 shown in FIG. 8 isrelatively long, and runs under constant tension over guide rollers 94,there being four such guide rollers in the apparatus of FIG. 8. Thepress belt B of the shoe press 91 shown in FIG. 9 is relatively short,and its path is a small circle.

[0004] In general, as shown in FIG. 10, the press roll P of each shoepress 91 includes both a cylindrical central portion, and end portionsP′ tapering in the axial outward direction. The shoe S of each shoepress 91 has chamfered round ends S′.

[0005] In the older shoe presses of the foregoing types, the nip zone N,that is, the zone underneath the middle portion of the press roll notincluding the tapered end portions P′, was wider than the shoe width S″,the shoe width being the width of the middle portion of the shoe, notincluding the chamfered round ends S′. On the other hand, in somecurrent shoe presses, as shown in FIG. 10, the nip zone N formed betweenthe roll P and the shoe S is narrower than the shoe width S″. The zonesabove the end portions of the shoe outside the nip zone N may bereferred to as non-nip zones N′.

[0006] In the shoe press shown in FIG. 10, a shearing force actscontinuously on the press belt B at the boundaries P″, between the nipzone N and the non-nip zones N′, at the inner ends of the tapered endportions P′ of the press roll. As a result of the shearing force, cracksare liable to develop along the boundary P″ in the belt B. These cracksadversely affect the durability of the press belt, making it unsuitablefor long-term use.

[0007] In view of the foregoing problem, the object of this invention isto provide a durable shoe press belt, in which the shearing force actingat the boundary between the nip zone and the non-nip zones is eithernon-existent or sufficiently weak that it causes minimal damage to thepress belt.

[0008] In accordance with the invention, a specially constructed shoepress belt is used in a shoe press which comprises a press roll and ashoe, and in which the nip zone formed between the press roll and theshoe is narrower than the shoe. In this press, therefore, there is a nipzone, and at least one non-nip zone, meeting at a boundary, which willordinarily coincide with the outer end of a cylindrical part of thepress roll. The press belt, which extends through the nip zone and thenon-nip zone, has a body comprising a hard portion and a portion of lesshardness, and a part of the portion of the belt having less hardness isdirectly opposite the boundary between the nip and non-nip zones.

[0009] In one preferred embodiment of the invention, the hard portion ofthe belt body comprises a high molecular weight elastic material, andthe portion of the belt body having less hardness comprises a highmolecular weight elastic material having a hardness less than that ofthe high molecular weight elastic material of the hard portion of thebody.

[0010] In another preferred embodiment, the hard portion of the bodycomprises a dense, high molecular weight, elastic material, and theportion of the body having less hardness comprises a high molecularweight elastic material having voids formed therein.

[0011] In still another embodiment, the belt body comprises a thinportion and a thick portion, so that a part of the thin portion can bepositioned directly opposite the boundary between the nip and non-nipzones.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a schematic cross section of the main portion of a beltembodying the invention;

[0013]FIG. 2 is a cross section of the main portion of a belt embodyingthe invention which includes a hard inner portion and an outer portionof less hardness, the outer portion extending outward from a pointslightly inward relative to the adjacent end of the cylindrical part ofthe press roll;

[0014]FIG. 3 is a cross section of the main portion of a belt embodyingthe invention, which includes a hard inner portion and an outer portionof less hardness, the outer portion extending outward from a pointcoinciding with the adjacent end of the cylindrical part of the pressroll;

[0015]FIG. 4 is a cross section of the main portion of a belt embodyingthe invention, which includes a hard inner portion and a portion of lesshardness, extending from a point slightly inward relative to theadjacent end of the cylindrical part of the press roll, to the outer endof the cylindrical part of the press roll;

[0016]FIG. 5 is a cross section of the main portion of a belt embodyingthe invention, and including a thin outer portion, the belt including atapered portion extending outward from a point located inward relativeto the adjacent end of the cylindrical part of the press roll;

[0017]FIG. 6 is a cross section of a main portion of a belt embodyingthe invention, and including a thin outer portion, the belt including atapered portion extending outward from a point coinciding with theadjacent end of the cylindrical part of the press roll;

[0018]FIG. 7 is a cross section of the main portion of a belt embodyingthe invention, and including a tapered portion which extends from apoint located inward relative to the adjacent end of the cylindricalpart of the press roll. The outer edge of the tapered portion is also onthe inside relative to the adjacent end of the cylindrical part of thepress roll;

[0019]FIG. 8 is a schematic view of the main portion of a shoe pressincorporating a long type shoe press belt;

[0020]FIG. 9 is a schematic view of the main portion of a shoe pressincorporating a short type shoe press belt; and

[0021]FIG. 10 is a schematic view showing the relationship of a pressroll, a shoe and a shoe press belt.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] Embodiments of the invention are described below with referenceto FIGS. 1-7.

[0023] Each of the belts in accordance with the invention is designedfor use with one of the shoe presses 91, shown in FIGS. 8 and 9, inwhich the nip zone N, formed between the press roll P and the shoe S, isnarrower than the shoe.

[0024] Each belt includes a body 11, as shown in FIG. 1, and, in each ofFIGS. 1-4, the belt body 11 comprises a hard portion K, and a portion Yhaving a hardness less than that of portion K. In a typical shoe pressbelt in accordance with the invention, the hard portion K is a centralportion, and the belt has two portions K, having less hardness, the twoportions extending along the central portion in side-by-siderelationship with the central portion. The portion Y is positioned sothat a part of the portion Y is directly opposite the nip end, at theboundary P″ between the nip zone N and the non-nip zone N′.

[0025] In each of FIGS. 5-7, the belt body 11 comprises a thin portion Hand a thick portion T. The thin portion H is positioned so that a partof the thin portion is directly opposite the nip end, at the boundary P″between the nip zone N and the non-nip zone N′. Although the thickportion T is uniform in thickness, the thin portion H varies inthickness, and includes all, or at least part of, a tapered transitionconnecting the outer part of the thin portion with the thick portion.

[0026] As shown in FIG. 1, the belt body 11 includes a base body 12 anda high molecular weight elastic material 13. The elastic material 13 maybe rubber or another elastomer, and is preferably a polyurethane resin.The base body 12 imparts strength to the belt body 11 and may be any oneof a number of known structures. For example, the base body 12 may be awoven fabric having a warp and weft, a fabric formed of a warp and weftsuperimposed on each other, or a fabric formed from a ribbon of clothwound in a spiral.

[0027] The belt body 11 can be produced by a process in which both sidesof the base body 12 are coated and filled with a high molecular weightelastic material 13. The upper and lower layers of the elastic material13 are joined to each other at the filling yarns (not shown) in the basebody 12, and cover both sides of the base body 12. After the elasticmaterial 13 cures, water holding grooves (not shown) may be formed inthe surface of the outwardly facing elastic layer of the belt.

[0028] In accordance with a recently proposed alternative process, thebelt body 11 may be manufactured by filling the base body 12 with highmolecular weight elastic material 13 from only the upper side of thebase body, while causing the elastic material to accumulate on bothsides of the base.

[0029] Alternatively, the belt may have a high molecular weight elasticmaterial 13 only on the press roll side of the base fabric so that basebody is exposed on the shoe side. In another alternative, the belt mayhave a high molecular weight elastic material 13 on the press roll sideof the base fabric, while the shoe-facing side may have a high molecularweight elastic material almost flush with the shoe-facing surface of thebase fabric.

[0030] In FIG. 1, the belt body 11 comprises a hard portion K and aportion Y having less hardness than that of portion K. The less hardportion Y extends through distance “a”, spanning the nip end of thepress roll P at boundary P″.

[0031] The difference in hardness between the less hard and hardportions Y and K depends on the selection of the high molecular weightelastic material. Where the belt body 11 is used at a relatively hightemperature of 50° C. or more, the less hard and hard portions Y and Kmay be resins that soften with heat to different degrees. The differencebetween the degrees of softening of the two portions of the belt resultsin a difference in hardness between the portions Y and K when the beltis in operation. Alternatively, the portions Y and K may be made fromidentical materials or from high molecular weight elastic materialshaving identical properties, including hardness. The portion Y may bemade less hard by the presence of air bubbles or other voids, while thehard portion K is more dense. In this case, a difference in themechanical structure between the portions Y and K of the belt bodyresults in a difference in hardness.

[0032] The high molecular weight elastic material may be formed with airbubbles or other voids in it by including hollow fillers or hollowmicrocapsules in the material, by including a foaming agent in thematerial, or by introducing air bubbles into the material by means of anair mixing machine.

[0033] The diameters of the air bubbles or other voids vary with thetype and hardness of the selected high molecular weight elasticmaterial, the function of the belt, and other factors. In general, it ispreferred that the diameters range from 10 to 100 μm. Experiments haveverified that, where hollow fillers or the like are used to produce thevoids in the less hard portion Y, the optimum ratio of the voids to thehigh molecular weight elastic material is achieved where the hollowfillers constitute from about 0.5 to 50 percent by weight of thematerial of the portion Y. In the case of air bubbles, experiments haveverified that the bubbles should be present in an amount in the rangefrom approximately 0.2 to about 30 percent by volume of the material ofthe portion Y.

[0034] It is preferable that the voids in the less hard portion Y beindependent of each other, so that cracks do not propagate from one voidto the other voids. Where the independent voids are formed by the use ofhollow fillers or hollow microcapsules, the voids are more uniform insize, and consequently, a consistent quality can be more easily achievedin the less hard portion Y.

[0035] The belt body 11 may comprise an outer less hard portion Y, and ahard portion K positioned within the nip zone N of the press roll P.Alternatively, the belt body 11 may comprise a first hard portion K, aless hard portion Y and a second hard portion K′. The first hard portionK is inside the nip zone N, while at least part of the second hardportion K′ is within the non-nip zone N′. The latter form of belt ispreferred for use in a shoe press requiring that the belt edges bestrong when the belt is fitted to the press.

[0036] It is preferable that the hard portions K and K′ should have ahardness between 85° and 98° (JIS-A). It is also preferable that theless hard portions Y should have a hardness between 80° and 95° (JIS-A).The difference in hardness between the hard and less hard portions K(and K′) and Y should preferably range between about 1 and about 5degrees. These preferred ranges of hardness and the preferreddifferences between the hardnesses of the hard and less hard portionshave been verified by experiment.

[0037]FIG. 2 shows a belt comprising an outer, less hard portion Y andan inner, hard portion K. The boundary between the hard portion K andthe less hard portion Y of the belt is located at a point Pn to theinside of the nip end boundary P″ of the press roll P. Thus, the lesshard portion Y spans the boundary P″ at the adjacent nip end.

[0038]FIG. 3 shows another belt comprising an outer, less hard, portionY and an inner, hard portion K. The boundary between the hard portion Kand the less hard portion Y of this belt coincides with the nip endboundary P″ of the press roll P. This belt is another embodiment inwhich the less hard portion Y is directly opposite the nip end boundaryP″.

[0039]FIG. 4 shows a belt comprising a first hard portion K, a less hardportion Y, and a second hard portion K′. The first hard portion K is tothe inside on the belt relative to the less hard portion Y. The boundarybetween the first hard portion K and the less hard portion Y of thisbelt is located at a point Pn to the inside of the nip end boundary P″.The boundary between the less hard portion Y and the adjacent secondhard portion K′ coincides with the adjacent nip end boundary P″.

[0040] In the embodiments shown in FIGS. 1-4, each of the hard and lesshard portions K (and K′) and Y may be uniform in hardness.Alternatively, there may be hardness gradients at the boundaries betweenthe portions K (and K′) and Y.

[0041] FIGS. 5-7 show belt bodies 11 each comprising an outer thinportion H and an inner thick portion T, where part of the thin portionis directly opposite to the nip end boundary P″ between the nip zone Nand the non-nip zone N′.

[0042] In FIG. 5, the inner thick portion T extends to a point Pn, whichis to the inside of the adjacent nip end boundary P″ of the press rollP. From point Pn, the belt is tapered at D from an edge D1 to an edgeD2. From edge D2 outward, the belt is of a uniform thickness less thanthat of thick portion T. The outer thin portion H includes not only theouter portion of uniform thickness, but also a part of the taperedportion of the belt. In this embodiment, the belt is out of directcontact with the nip end of the belt.

[0043] In FIG. 6, the belt includes a tapered portion D between theinner thick portion T and an outer thin portion of uniform thickness.The tapered portion D extends from edge D1, which coincides the adjacentnip end boundary P″ of the press roll P, to edge D2. In this case, thethin portion of the belt includes the entire tapered portion. Becausethe tapered portion D begins at a point coinciding with the adjacent nipend boundary P″, a shearing force exerted by the roll P will have lesseffect on the belt than it would if the belt were of uniform thicknessthroughout its width.

[0044] In FIG. 7, the belt also includes a tapered portion D between theinner thick portion of the belt and an outer portion of uniformthickness less than the thickness of the inner thick portion. Thetapered portion D extends from edge D1, at a point Pn to the inside ofthe adjacent nip end boundary P″, to edge D2. This belt is similar tothat shown in FIG. 5, in that the belt is out of direct contact with thenip end of the belt. However, the belt of FIG. 7 differs from the beltof FIG. 5 in that the edge D2 of the tapered portion D in FIG. 7 is tothe inside of the nip end boundary P″. Accordingly, the distance “b”between the thin outer portion of the belt and the adjacent nip endboundary P″ in FIG. 7 is greater than that in FIG. 5.

[0045] The angle of rake of the tapered portion D may be variedaccording to the hardness of the selected high molecular weight elasticmaterial, and according to other factors, but preferably falls withinthe range of 0.4 and 0.5 degrees. It has been verified by experimentthat the maximum thickness difference between the edges D1 and D2 of thetapered portion D is preferably about 1.5 mm.

[0046] Another belt (not shown) embodying the invention comprises acentral thick portion, a thin portion in the form of a recess to theoutside of the thick portion, and another outer thick portion outsidethe outer thin portion. The recess should be located so that a part ofthe recess is directly opposite the nip end boundary P″.

[0047] The shoe press belt according to the invention, which ischaracterized by a body comprising a hard portion and a portion of lesshardness, a part of the less hard portion being directly opposite theboundary between the nip and non-nip zones, has the outstandingadvantage that it does not tend to form cracks even though it issubjected to a continuous shearing force due to the fact that the nipzone width is less than the shoe width.

[0048] The less hard portion of the belt, which avoids the adverseeffects of the shearing force applied by the nip end of the roll, can beproduced easily by utilizing different high molecular weight elasticmaterials for the hard and less hard portions of the belt.

[0049] Alternatively, the difference in hardness between the twoportions of the belt can be achieved by utilizing a dense high molecularweight elastic material for the hard portion of the belt and forming theless hard portion from a high molecular weight elastic material havingvoids formed therein. In this case, the less hard portion, which avoidsthe adverse effects of the shearing force applied at the nip end of thepress roll, can be easily produced mechanically by filling the portionof the belt which is to be less hard with air bubbles or other voids.

[0050] Finally, a similar effect can be achieved by forming the belt sothat it comprises a thin portion and a thick portion, so that a part ofthe thin portion can be located directly opposite to the boundarybetween the nip and non-nip zones. In this case, the belt is either outof direct contact with the nip end of the press roll or, if in contact,not subject to severe shearing forces.

I claim:
 1. A shoe press belt for use in a shoe press having a nip zonenarrower than the shoe, and at least one non-nip zone meeting the nipzone at a boundary, the belt having a body comprising a hard portion anda portion having less hardness, said portions being in side-by-siderelationship, whereby a part of the portion of the belt having lesshardness can be positioned directly opposite the boundary between thenip and non-nip zones.
 2. A shoe press belt according to claim 1,wherein the hard portion of said body comprises a high molecular weightelastic material, and the portion of said body having less hardnesscomprises a high molecular weight elastic material having a hardnessless than that of the high molecular weight elastic material of the hardportion of the body.
 3. A shoe press belt according to claim 1 whereinthe hard portion of said body comprises a dense, high molecular weight,elastic material, and the portion of said body having less hardnesscomprises a high molecular weight elastic material having voids formedtherein.
 4. A shoe press belt for a shoe press including a press rolland a shoe, the press having a nip zone and a non-nip zone, the nip zonebeing formed between the roll and the shoe and being narrower than theshoe, the belt having a body comprising a thin portion and a thickportion, whereby a part of the thin portion can be positioned directlyopposite the boundary between the nip and non-nip zones.
 5. A shoe presscomprising: a press roll and a shoe, the press having a nip zone and atleast one non-nip zone, the nip zone formed between the press roll andthe shoe, and being narrower than the shoe, and the nip zone and thenon-nip zone meeting each other at a boundary; and a press beltextending through the nip zone and the non-nip zone; in which the belthas a body comprising a hard portion and a portion of less hardness, andin which a part of the portion of the belt having less hardness isdirectly opposite the boundary between the nip and non-nip zones.
 6. Ashoe press according to claim 5, wherein the hard portion of said bodycomprises a high molecular weight elastic material, and the portion ofsaid body having less hardness comprises a high molecular weight elasticmaterial having a hardness less than that of the high molecular weightelastic material of the hard portion of the body.
 7. A shoe pressaccording to claim 5 wherein the hard portion of said body comprises adense, high molecular weight, elastic material, and the portion of saidbody having less hardness comprises a high molecular weight elasticmaterial having voids formed therein.